For determining physical, measured variables, widely varying types of sensors are used, which can be based on very different, physical measuring principles. Thus, for instance, the fill level of a fill substance in a container can be measured on the basis of the travel time of ultrasonic waves or electromagnetic waves, especially microwaves, reflected from the surface of the fill substance. In the case of microwaves, these are either radiated freely into the container in the direction of the surface of the fill substance, or the microwaves are guided into the container along a conductive element. Besides the use of these principles of measurement, capacitive and radiometric measuring methods are also used for fill level measurement.
For detecting the limit level of a fill substance in a container, preferably the resonance frequency of an oscillating rod, or of an oscillatable structure constructed from a plurality of rods, is evaluated. In the case of this measuring method, use is made of the effect that the resonance frequency changes, depending on whether the oscillating rods oscillate freely or else are in contact with the fill substance. Corresponding measuring devices are available from the assignee of the present application in many possible variants.